This invention relates to an exhaust emission control device of an internal combustion engine, which has an occlusion type NOx catalyst in an exhaust passage.
In recent years, a lean burn internal combustion engine designed to be operated at a lean air-fuel ratio for improved fuel economy has come into practical use. When this lean burn internal combustion engine is operated at a lean air-fuel ratio, a three-way catalyst cannot fully purify NOx (nitrogen oxides) in an exhaust gas because of the purification characteristics of the catalyst. Recently, for example, use has been made of an exhaust emission purification catalyst device with an occlusion type NOx catalyst which occludes or adsorbs (hereinafter referred to simply as occludes) NOx in an exhaust gas during operation at a lean air-fuel ratio, and which releases and reduces the occluded NOx during operation at a stoichiometric air-fuel ratio or a rich air-fuel ratio.
The occlusion type NOx catalyst has the properties of attracting and occluding NOx in an exhaust gas as nitrates (Xxe2x80x94NO3) in the presence of excess oxygen in the internal combustion engine, and releasing the occluded NOx mainly in the presence of excess carbon monoxide (CO) to reduce it to nitrogen (N2) (and simultaneously form carbonates (Xxe2x80x94CO3)). In actuality, therefore, when the operation at a lean air-fuel ratio continues for a predetermined period of time, the air-fuel ratio in a combustion chamber is changed. Alternatively, the operation at the lean air-fuel ratio is periodically switched to operation at a rich air-fuel ratio by such means as supply of a reducing agent to an exhaust pipe to control the air-fuel ratio to a stoichiometric air-fuel ratio or a rich air-fuel ratio (this method is called rich spike). As a result, a CO-abundant reducing atmosphere is produced in an oxygen concentration-poor atmosphere to release the occluded NOx for purifying reduction (NOx purge). In this manner, the occlusion type NOx catalyst can be regenerated. This technology is disclosed, for example, in Japanese Patent No. 2,586,738.
Such an occlusion type NOx catalyst is limited in the amount of NOx that can be occluded onto the catalyst. When the amount of NOx, which the occlusion type NOx catalyst has occluded, reaches the limit, rich spike is performed as stated earlier, so that a rich air-fuel ratio operation is carried out at a predetermined rich air-fuel ratio for a predetermined period of time.
The time point, at which the occluded amount of NOx in the occlusion type NOx catalyst reaches the limit and rich spike is required, changes according to the degree of deterioration in the NOx purification efficiency of the occlusion type NOx catalyst, or according to the operating conditions for the internal combustion engine that affect the flow rates of NOx and CO. A technology for detecting that the amount of NOx occluded by such an occlusion type NOx catalyst has reached the limit, and regenerating this catalyst is disclosed, for example, in Japanese Unexamined Patent Publication No. Hei 7-166851.
According to an xe2x80x9cexhaust emission control devicexe2x80x9d as disclosed in this publication, an NOx absorber is disposed in an exhaust passage of an internal combustion engine, and an NOx sensor is disposed downstream from the NOx absorber. With this arrangement, the exhaust emission control device performs regenerating control (NOx purge) in the following manner: When a detection value detected by the NOx sensor (i.e., the concentration of the NOx component) has exceeded a judgment value, the exhaust air-fuel ratio is switched to a rich air-fuel ratio to release NOx from an upper region of the catalyst.
The regulatory value according to the NOx emission regulations in different countries of the world is, for example, total NOx discharge over a predetermined distance traveled by a vehicle. The aforementioned conventional xe2x80x9cexhaust emission control devicexe2x80x9d executes regenerating control by detecting the concentration of the NOx component during each lean operation zone which is a certain short period. Thus, if the set judgment value allows only a narrow margin because of a driver""s way of driving, it is likely that the desired NOx discharge, e.g., the regulatory value, for each predetermined distance traveled cannot necessarily be cleared reliably.
That is, in the above-mentioned xe2x80x9cexhaust emission control device,xe2x80x9d whether the total NOx discharge over the predetermined distance traveled has not exceeded a desired value, e.g., the regulatory value, is not grasped during driving. To keep the total NOx discharge over the predetermined distance traveled at a predetermined value or less under various driving conditions, therefore, there is need to set the aforementioned judgment value for initiation of regenerating control (NOx purge) at a low value with a sufficient margin. By setting the judgment value in this manner, the frequency of regenerating control (NOx purge) for making the air-fuel ratio a rich or stoichiometric value increases because of an ample margin, thereby increasing fuel consumption. That is, the problem of increasing CO2 discharge arises.
A technology for controlling NOx discharge over a predetermined distance traveled to be within a predetermined value is disclosed, for example, in Japanese Patent No. 2,503,387. An xe2x80x9celectronic internal combustion engine control devicexe2x80x9d as disclosed in this publication controls the ignition timing and the EGR amount to control NOx discharge, only in a stoichiometric operation region. If this control device is applied to a lean burn internal combustion engine operating at a lean air-fuel ratio as described earlier, the air-fuel ratio has to be constantly set in the stoichiometric operation region, and fuel economy cannot be improved.
The present invention has been accomplished to solve the above problems. Its object is to provide an exhaust emission control device of an internal combustion engine, which can directly manage NOx discharge, released into the atmosphere, under any driving conditions to keep the NOx discharge at a desired value reliably and achieve a decrease in NOx discharge and a decrease in CO2 discharge at the same time, without deteriorating fuel economy.
An exhaust emission control device of an internal combustion engine according to the present invention includes an exhaust emission purification catalyst device provided in an exhaust passage of the internal combustion engine, and having a NOx decreasing function of purifying or occluding NOx in an exhaust gas when an exhaust air-fuel ratio is a lean air-fuel ratio, and the reducing function of reducing hazardous substances in the exhaust gas when the exhaust air-fuel ratio is a stoichiometric air-fuel ratio or a rich air-fuel ratio; NOx detection means for detecting or estimating the concentration of NOx released into the atmosphere; and control means for computing an NOx discharge released into the atmosphere based on an output from the NOx detection means, and stopping or suppressing an operation at a lean air-fuel ratio when detecting or expecting that a total NOx discharge exceeds a predetermined value before a vehicle reaches a predetermined driving period.
Thus, when the exhaust air-fuel ratio is a lean air-fuel ratio, NOx in the exhaust gas is purified or occluded. A total NOx discharge is computed from the concentration of NOx released into the atmosphere, with a predetermined driving period as a cycle. If it is detected or expected that the total NOx discharge exceeds the predetermined value before the predetermined driving period is reached, operation at the lean air-fuel ratio is stopped or suppressed, and the reducing function of the exhaust emission purification catalyst device is performed. Under any driving conditions, therefore, poor fuel economy is not induced. Under any driving conditions, moreover, the NOx discharge can be restricted to the desired value. Thus, a decrease in NOx discharge and a decrease in CO2 discharge can be achieved at the same time.
In the exhaust emission control device of an internal combustion engine according to the present invention, the control means may change the exhaust air-fuel ratio to a stoichiometric air-fuel ratio or a rich air-fuel ratio when it detects that the total NOx discharge has exceeded the predetermined value before the vehicle reaches the predetermined driving period. By doing so, hazardous substances in the exhaust gas can be reduced, and the NOx decreasing function of the exhaust emission purification catalyst device can be regenerated immediately.
In the exhaust emission control device of an internal combustion engine according to the present invention, after changing the exhaust air-fuel ratio to the stoichiometric air-fuel ratio or the rich air-fuel ratio when detecting that the total NOx discharge has exceeded the predetermined value before the vehicle reaches the predetermined driving period, the control means may maintain the exhaust air-fuel ratio at the stoichiometric air-fuel ratio or the rich air-fuel ratio until the vehicle reaches the predetermined driving period. By doing so, NOx emission can be suppressed.
In the exhaust emission control device of an internal combustion engine according to the present invention, the control means may also decrease an operation region at the lean air-fuel ratio if the total NOx discharge is expected to exceed the predetermined value before the vehicle reaches the predetermined driving period. By doing so, the lean operation can be continued in an optimal driving state, with NOx emission being suppressed. Thus, fuel economy can be improved.
In the exhaust emission control device of an internal combustion engine according to the present invention, the control means may also change the operation region at the lean air-fuel ratio based on the total NOx discharge midway through the predetermined driving period. By doing so, the lean operation can be continued in an optimal driving state, with NOx emission being suppressed. Thus, fuel economy can be improved.
In the exhaust emission control device of an internal combustion engine according to the present invention, the control means may also reset the computation of the total NOx discharge and measurement of the predetermined driving period when the vehicle reaches the predetermined driving period. At this time point, control for NOx suppression is started.
In the exhaust emission control device of an internal combustion engine according to the present invention, if the total NOx discharge does not exceed the predetermined value even when the vehicle has reached the predetermined driving period, the control means may change the exhaust air-fuel ratio to a stoichiometric air-fuel ratio or a rich air-fuel ratio after the total NOx discharge exceeds the predetermined value, and then the control means may reset the computation of the total NOx discharge and measurement of the predetermined driving period. At this point in time point, control for NOx suppression is started.
In the exhaust emission control device of an internal combustion engine according to the present invention, if the duration of the operation at the lean air-fuel ratio exceeds a predetermined threshold time, the control means may change the exhaust air-fuel ratio to a stoichiometric air-fuel ratio or a rich air-fuel ratio. By doing so, the NOx decreasing function can be regenerated, with NOx emission being suppressed.
In the exhaust emission control device of an internal combustion engine according to the present invention, if the duration of the operation at the lean air-fuel ratio is a predetermined threshold time or more, and if the total NOx discharge or an average NOx concentration or an instantaneous NOx concentration is a predetermined value or more during the predetermined threshold time or more, the control means may change the exhaust air-fuel ratio to a stoichiometric air-fuel ratio or a rich air-fuel ratio.
In the exhaust emission control device of an internal combustion engine according to the present invention, the control means may change the predetermined value for the total NOx discharge according to the vehicle speed. This makes NOx emission suppressing control possible according to the driving state of the vehicle.
In the exhaust emission control device of an internal combustion engine according to the present invention, the exhaust air-fuel ratio may be changed to a stoichiometric air-fuel ratio or a rich air-fuel ratio in response to a driver""s acceleration action, and fuel injection into a cylinder in and after an expansion stroke may be used concomitantly at an initial stage of changing the air-fuel ratio. By doing so, NOx can be reduced at an early stage, so that the total NOx discharge can be decreased.